Time to replace galvanized where meeting copper?

I recently noticed some rusty deposits on the top of my water heater, so I looked up and discovered the galvanized pipes are connected directly to newer copper pipes. I understand this is not a good idea unless a dielectric union is used because corrosion occurs quickly. My question is, how "quickly" does it occur, and should I make it a priority to replace these fittings ASAP? Should I replace with dielectric unions, or should I replace ALL of the old galvanized pipes?

The galvanized plumbing is from 1951, so it's going on 60 years old. Most of it is exposed in the unfinished basement. I found about 4 places where the old galvanized pipe is fitted with newer copper piping, and attached some photos here. They all look like they are corroding. I'd rather not wait for a leak to develop, but I also don't want to be hasty in incurring the expense of replacing the galvanized pipes all the way back to the street if I don't need to. Is it better to replace with copper or PEX?

It's all up to the galvy fittings. If they are still in good condition, thread wise, then transition to either PEX or copper but the only way to find out is to start unthreading connections and see the condition of the pipes. Wouldn't hurt to have your plumber's opinion on site as well.

That buried galvanized has got to be about to fall apart! The interior can't be in very good shape either, but it's not being attacked from the outside. You are going to have to replace the pipes soon. How you go about it is up to you. You can pick a time when weather isn't too bad for working outside and do the outside only. Then replace the rest of it piece meal as it fails. But, realize Murphy's Law will apply about pipes failing at the worst possible time. Or, you can just bite the bullet and replace it all and be done with it.